Joints in tube fittings have been made in a number of different varieties and for many different uses. The typical joint is one wherein a tube is connected into a thick-walled metal body called a coupling or a fitting and then an orifice within this thick-walled body conveys the fluid to some utilization device or connects to still another tube for further transfer of the fluid. Such couplings or fittings are large, bulky, require considerable machining, and hence are relatively expensive.
Welded joints between two tubes have previously been used but this requires considerable equipment, either gas welding or electric welding. Further, such welding is not suitable for many metals, for example, aluminum. Aluminum is a metal which does not appreciably change color as it is heated so that it is difficult for the welder to determine when the metal has reached the proper welding temperature. If the welding temperature is just slightly exceeded, the metal rapidly droops and then runs away as a liquid. Still further, aluminum as it is heated very readily oxidizes and the aluminum oxide coating is a very good insulator which inhibits a satisfactory weld. This means that usually some shielded arc or inert gas atmosphere must surround the weld, thus still further complicating and increasing the cost of such weld.
Many tube joints are subjected to an adverse environment, for example, in the use of air conditioning assemblies in automobiles. The refrigerant used in the air conditioning units is rather corrosive and over a period of time appears to attack almost all packing or sealing material in the tube joints, thus eventually causing leaks. Additionally, the temperature range within the engine compartment of a modern automobile is as much as 200.degree. F., from below 0 to near 200 degrees. Still further, there is considerable vibration both from the engine and from road shocks. The combination of these factors has caused the use in automotive air conditioners of tube joints which are rather expensive to manufacture and to assemble, in man hours of labor. Additionally there are millions of such automotive air conditioners manufactured each year with many joints to be made for each air conditioner, and thus the industry is quite anxious to obtain a tube joint which is not only economical to use and manufacture but also reliable so that expensive warranty work is not needed. Retrofitting a defective joint in an existing automobile out in the field is far more expensive than the total cost of all fittings on the entire automotive air conditioner in the first instance. Hence the industry is not interested in a cheap fitting which does not maintain its integrity under the severe conditions encountered.
The air conditioning industry would also like to use aluminum tubing because it is lightweight yet strong, but aluminum has traditionally been a difficult metal to join with another structure, one reason being that it is initially ductile but rapidly becomes work hardened.
The prior art has known tube joints with a flange or an annular bead, including those fittings where such bead acted against a resilient packing member, or where a flared end on the tube coated with a coupling. However most of these fittings were reusable fittings of the two piece type such as an interthreaded nut and fitting connection. These are expensive to manufacture and time consuming to assemble. Where it is not necessary that the joint be disassembled after initial assembly, the prior art has known of fittings which are assembled by an internal mandrel expanding the tube against a wall of the fitting. In this case the outwardly swaged tube is moved into engagement with a previously prepared aperture in the thick-walled fitting. It takes several steps to prepare the proper shape of this fitting to receive the tube. Then a further three or four step process is required to use a mandrel and die to outwardly swage the tube and finish the process of connecting the tube to the thick-walled fitting. Accordingly, the problem to be solved is how to construct a tube joint and the method of assembling the same so as to overcome the economic and practical disadvantages of the prior art.